In the state of the art devices for producing milk foam are known, wherein the foaming of the milk is typically carried out in batch processes. For example, a device is known that has a reservoir, into which milk can be filled. A rotating part in the reservoir, for example a whisk provided at the bottom surface of the reservoir, causes the foaming of the milk. Such a device for producing milk foam, however, can produce only a predetermined amount of milk foam at the same time, i.e. in one batch process. Afterwards, the device needs to be emptied, preferably cleaned, and refilled with milk before the next batch process can be started. Furthermore, such a device cannot be implemented in-line of a fluid flow path, and thus e.g. in a device for producing or providing milk or other beverages.
Further, the state of the art includes devices that inject hot steam into milk that is filled into a reservoir, in order to cause foaming. However, such devices cannot be used in-line of a fluid flow path of e.g. a beverage producing device.
For the above-mentioned state of the art devices, the factors that influence the foaming of the milk are, for example, the geometry of the rotating part, like the whisk, or the temperature and/or pressure of the steam that is injected into the milk. These factors are difficult to understand, and are not easy to control accurately without building more complicated devices. Therefore, the milk foam of many simple state of the art foaming devices is often produced unreliably, i.e. the properties of the foam like volume, foaming level, foam stability etc. differ from one batch process to another.